This project is designed to evaluate the potential role of Vitamin D3 analogs in breast and prostate cancers. The GCRC collects biopsies from the operating room when patients are undergoing mastectomy for cancer. The cells obtained from these biopsies are then grown in mice and treated with Vitamin D3 analogs alone or in combination with other agents to determine if the various regimens diminish the growth capability of the tumor cells. During the coming year the GCRC will continue to collect tumor samples, particularly prostate cancer specimens.